Avoid a hang when probing the partition table on imported block

Avoid a hang when probing the partition table on imported block
devices.  If you export a device so the importing domain sees it as a
whole disk rather than a partition, then add_disk will try to probe
its partition table.  However, we were calling add_disk before the
device was properly connected, and then not connecting it until
add_disk had finished.  This meant that we ended up never actually
connecting the device, and any accesses to it would then hang.

Fix this by not calling add_disk until we're connected.

Signed-off-by: Steven Smith, sos22@cam.ac.uk